Touch sensing systems (“touch systems”) are in widespread use in a variety of applications. Typically, the touch sensing systems are actuated by a touch object such as a finger or stylus, either in direct contact, or through proximity (i.e. without contact), with a touch surface. Touch sensing systems are for example used as touch pads of laptop computers, in control panels, and as overlays to displays on e.g. hand held devices, such as mobile telephones and interactive whiteboards. A touch panel that is overlaid on or integrated in a display is also denoted a “touch screen”. Many other applications are known in the art.
WO2010/064983 discloses a multi-touch system, which is based on frustrated total internal reflection (FTIR). Light sheets are coupled into a panel to propagate inside the panel by total internal reflection (TIR). When an object comes into contact with a touch surface of the panel, the propagating light will be attenuated at the point of touch. Arrays of light sensors are located around the perimeter of the touch surface to detect the received light for each light sheet and generate a signal value for each light path across the touch surface. The signal values of the light sensors are normalized by respective reference values and input into an image reconstruction algorithm that generates a two-dimensional distribution of attenuation values across the touch surface. This enables repeated determination of current position/size/shape of touches while one or more users interact with the touch surface. In FTIR systems, the touches need to be detected against a background of interferences, e.g. originating from fingerprints and other types of smear on the touch surface. The influence of interferences may vary not only over time but also across the touch surface, making it difficult to properly detect the touches on the touch surface at all times. WO2011/028169 proposes manipulating the reference values to compensate for the influence of contaminations in the attenuation pattern, and WO2011/049512 proposes tracking a two-dimensional background pattern of the contamination contribution and using the background pattern to generate an attenuation pattern that is compensated for contaminations.
Optical touch systems often need to be designed with a high sensitivity, since a touching object may result in only a small attenuation of the propagating light, e.g. less than 1%. Certain systems may therefore be designed to detect attenuations on the order of 0.1%-0.01%. To ensure proper touch detection, WO2011/078769 proposes monitoring the temporal variability of the signal values to identify potential causes for reduced performance of the touch system, e.g. due to component failure, and implementing corrective measures to account for unreliable signal values.
WO2013/115710 describes processing the output signal of an optical touch system to generate a set of data samples comprises ON energy values when the light signal is being transmitted and OFF energy value when no light signal is being transmitted. An estimated ambient value is determined from the OFF energy value. An ambient-compensated energy value is generated by subtracting an estimated ambient value from the ON energy value for each actual detection line. However, this technique only provides significant improvements for light noise sources that are unmodulated (e.g. Sunlight) or modulated with a low frequency (e.g. Incandescent lamps). This technique provides poor results with respect to modulated noise sources like fluorescent lighting.
Therefore, what is needed is a way of improving performance of an optical touch system in an environment that comprises various types of ambient light noise.